I read on a site about a month ago that the low pressure switch on the A/C is different for the 870 computer vs. the 165. I'm running an 89 chip in an adapter harness and when a mechanic looked at the car on his machine it said the A/C was on all the time, and it wasn't on....

Seems the individual that made the statement about the switch said it had to do with a setting in the chip. If this is true, would it cause my A/C to overpressure???? I have had the worst time blowing seals on compressor units..... Are there any 1989 350 TPI chips that are set to use with the 1985 style low pressure seal or do I need to pipe in a 'T' and hook them both up before I charge the system this time?

I also intend to check out that adapter harness now that I have the wiring diagrams, thanks to this site.... BTW, what I have looked at on the wiring diagrams doesn't coincide with what I am seeing on some of the "Conversion" diagrams floating around on this site. If I had a web to post from, I would show you guys the difference I am seeing....

 

The pressure switch is actually mechanical on most of the models as far as I know. Dont know much about which ecms can control the A/C though. If that is so, then having the compressor on at all times is definatly bad for it. Without air going through the evaporator, eventually the compressor can actually get a vapor/liquid mix of refirgerant from the condensor since little heat is put into the system, which is hard as hell on it. The compressor has to compress a liquid/gas mixture instead of jsut a gas.

Hmmm... interesting about using an 89 chip. Didnt know the P4 ecm came with some of the 85 models. Thought it was only out in 86 and above.

 

Easy way to tell if it continues to run is to look at the clutch assy. on the ac compressor. If the part in the center with the 3 metal bits on it in a triangle is turning, then the clutch is engaged and the compressor is on.

 

Clarification: Some years ago I had a guy at Fuel Injection Specialties build an adapter harness so I could run the 165 computer in my 85 without having to modify my existing harness. At the time this was my only car and I couldn't have it down long enough to modify the harness and work through any bugs. Especially since I didn't have access to any wiring diagrams....

I don't recall why he (F.I.S.) recommended using the 89 chip instead of the 86-88 chip. I do know that the cold start injector doesn't work with this adapter and the fan is supposed to stay on at all times.

The fan actually turns off about a minute or so after the AC is turned on so I wired in the fans (yeah, added a few extras) a little differently. I have a 14" and a 10" in front of the Condensor. The 10" and the 14" are controlled by the computer. Behind the radiator I have a 16" and a 10" fan. The 10" runs all the time and the 16" is run by a thermostatic fans switch in the head that is set to come on at 205 and shut off at 190.

The AC is not on all the time, the computer is "thinking" the AC is on at all times. IIRC the article/thread I read said that the 85 low pressure switch is normally open and the 86-89 switch is normally closed although some are normally open........ Hence the problem....

I just replaced the AC unit right before I trashed the transmission and I haven't charged it yet. Albeit only because I have another problem I have to figure out. Even though I jumpered the low pressure switch so I could charge it, the clutch on the compressore never engaged. I'm thinking I may have trashed that switch in the top of the compressor when I removed it from the old compressor. Heck, I don't even know what that switch is for.....

What I "think" the computor controls on the AC unit is the fans and the compressor engagement of the clutch. There is a temperature switch in the line and when it reaches a certain temperature it turns on/off the fan(s). I don't know but what that switch in the compressor is a pressure switch on the high side, that may be affected by the computer controls as well.

Any ideas guys??? BTW,do we maybe need an air-conditioning/heater board????? Ha! Ha!

:hail:

 

Im not too familiar with the older f-bodies so Ill leave it for someone else. Might even do better on the tech/general engine board.

On the newer ones, though, the a/c clutch is run off of the HVAC control head so maybe you could wire it up like it is on the newer cars. Probably would take some of the headache out of using the ac. If what you said about the pressure switch is true, youd need one that was notmally closed to run it like this so the a/c turns on when the HVAC controls are moved to the position for a/c.